1. Field of the Invention
The present invention relates to a substrate having a buried structure that serves as an interconnect, a black matrix (light blocking film) or a waveguide. The present invention also relates to a display device including such a substrate, a method of making the substrate and a method of fabricating the display device.
2. Description of the Related Art
Various types of display devices currently under development, including liquid crystal display devices and organic EL display devices, have been reducing their thicknesses and increasing their definition or screen size day after day. These display devices with reduced thicknesses may be driven by various techniques. Among other things, an active-matrix addressing technique contributes particularly effectively to the improvement of display definition.
An active-matrix addressed display device includes multiple active components (e.g., TFTs or MIMs), each being provided for an associated one of its pixels. In a display device of this type, the optical state of a pixel is changeable by way of its associated active component. Each active component is formed on a glass substrate and connected to a gate line and a source line. For example, a TFT, which is provided for associated one of pixels that are arranged in columns and rows (i.e., in matrix), is connected to a gate line (or scan line) and a source line (or data line) that intersect with each other.
A glass substrate, on which multiple active components are formed in matrix in this manner (which is often called an xe2x80x9cactive-matrix substratexe2x80x9d), normally has a structure in which multiple gate lines and multiple source lines cross each other. For example, an active-matrix substrate may be formed in such a manner that the source lines extend over, and overlap with, the gate lines. In that case, although an insulating film is interposed between each gate line and associated one of the source lines, the source line still has to get over level differences that are created by the gate line.
Accordingly, to prevent the source line from being discontinued by the level differences created by the gate line or to keep the orientation state of liquid crystal molecules in a liquid crystal display device from being disturbed by those level differences on the surface of the substrate, the level differences need to either reduce its height (or thickness) or decrease the angle that the level differences define with respect to the surface of the substrate. On the other hand, the gate lines and source lines should have resistance that is low enough to transmit a predetermined electric signal at a sufficiently high rate. To satisfy these requirements, interconnects, including the gate and source lines, on a conventional active-matrix substrate have their thickness limited (e.g., to 0.3 xcexcm or less) so as not to be discontinued (or disconnected) by those level differences and also have their width adjusted so as to reduce their resistance sufficiently. The interconnects sometimes have tapered side surfaces for these purposes.
In this manner, according to the conventional techniques, an interconnect having a limited thickness should have its width adjusted to obtain a desired resistance value. That is to say, the conventional techniques allow only a low degree of flexibility for the interconnects being designed. Thus, in a transmission type liquid crystal display device, for example, the aperture ratio thereof has to be sacrificed to make the interconnects satisfy those requirements. It is already known that a display device having a diagonal size of greater than 10 inches has its aperture ratio adversely limited by such an interconnection structure.
In a transmission type liquid crystal display device, however, as its aperture ratio decreases, the luminance of the image displayed thereon decreases or the power dissipation thereof increases. Accordingly, to enhance the performance of a transmission type display device, interconnects having a narrow line width and a sufficiently low resistance value need to be realized.
To overcome these problems, a method of reducing the thickness of the level differences on the substrate and realizing interconnects with a sufficiently low resistance value was proposed in Japanese Laid-Open Publication No. 4-170519, for example. According to this proposed method, an interconnect, having a thickness greater than that of the conventional one, is embedded in a groove that has been formed on the surface of a substrate. An interconnect of this type will be herein referred to as an xe2x80x9cinlaid interconnectxe2x80x9d.
However, the technique of forming such an inlaid interconnect on the surface of a glass substrate has not been established yet. For that reason, Japanese Laid-Open Publication No. 4-170519 identified above provides no disclosure about a specific method of forming the groove on the surface of a glass substrate. The present inventors discovered and confirmed via experiments that when a glass substrate was wet-etched by using a conventional etchant as a mixture of hydrofluoric acid and ammonium fluoride, the surface could not be etched uniformly or at a sufficiently high rate or might have a groove with a width that had been unnecessarily broadened by an abnormal side-etching phenomenon. If the surface of a glass substrate is etched non-uniformly, then the etched surface will scatter light excessively to make the displayed image whitened.
In order to overcome the problems described above, preferred embodiments of the present invention provide a substrate having a buried structure such as the inlaid interconnect on the glass substrate and also provide a display device including such a substrate, a method of making the substrate, and a method of fabricating the display device.
A preferred embodiment of the present invention provides a method of making a substrate having a buried structure. The method preferably includes the steps of preparing a glass substrate having a principal surface, forming a groove on the principal surface of the glass substrate by a wet etching process, and depositing a first material over the principal surface of the glass substrate and filling the groove with the first material to form the buried structure having a surface that is substantially flush with the principal surface. The step of forming the groove preferably includes the step of performing the wet etching process by using an etchant that includes hydrofluoric acid, ammonium fluoride, and hydrochloric acid or oxalic acid.
In one preferred embodiment of the present invention, the step of forming the groove preferably includes the step of forming the groove that satisfies a relationship dxe2x89xa7r, where d is the depth of the groove and r is a radius of curvature of a sidewall of the groove. Specifically, the groove may have a depth d of 0.5 xcexcm or more.
In another preferred embodiment of the present invention, the step of preparing the glass substrate preferably includes the step of preparing the glass substrate that is mainly composed of silicon dioxide and that additionally includes a non-silicon-dioxide metal oxide. Specifically, the glass substrate may be made of either non-alkali glass or soda lime glass.
In still another preferred embodiment, the step of forming the groove preferably includes the step of defining an etching mask that exposes a region of the principal surface of the glass substrate in which the groove will be formed. The step of forming the buried structure preferably includes the step of removing the first material from the principal surface of the glass substrate, except a portion of the first material that fills the groove, by a lift-off process using the etching mask.
In this particular preferred embodiment, the step of forming the groove preferably further includes the step of subjecting the principal surface of the glass substrate to a surface treatment to make the etching mask contact with the principal surface more closely before the step of defining the etching mask is performed.
More particularly, the step of forming the groove preferably includes the steps of subjecting the principal surface to a silylation process as the surface treatment and defining a photoresist pattern as the etching mask on the principal surface.
In yet another preferred embodiment, the step of depositing the first material preferably includes the step of depositing the first material by a sputtering process.
In yet another preferred embodiment, the step of depositing the first material may include the steps of depositing a conductive material as the first material and forming an interconnect as the buried structure. Alternatively, the step of depositing the first material may include the step of depositing a light blocking material as the first material and forming a black matrix as the buried structure.
As another alternative, the step of depositing the first material may include the steps of depositing a transparent material as the first material and forming a waveguide as the buried structure.
Another preferred embodiment of the present invention provides a method for fabricating a display device that includes an active-matrix substrate and a display medium layer. The method preferably includes the step of making the active-matrix substrate by the method of making the substrate with the buried structure by using the conductive or light blocking material.
In one preferred embodiment of the present invention, the buried structure may be a gate line. In that case, the step of making the active-matrix substrate preferably includes the step of forming a reverse staggered TFT on the gate line.
In an alternative preferred embodiment, the buried structure may be a source line. In that case, the step of making the active-matrix substrate preferably includes the step of forming a staggered TFT on the source line.
A substrate according to still another preferred embodiment of the present invention preferably includes a glass plate having a groove that has been formed on the principal surface thereof, and a buried structure. The buried structure is preferably made of a first material that has been deposited on the principal surface so as to fill the groove and preferably has a surface that is substantially flush with the principal surface. The groove preferably satisfies a relationship dxe2x89xa7r, where d is the depth of the groove and r is the radius of curvature of a sidewall of the groove.
In one preferred embodiment of the present invention, an inner surface of the groove preferably has a roughness that is one-tenth or less of the depth d of the groove. This is because the present inventors discovered and confirmed via experiments that when the inner surface of the groove had a roughness greater than this value, the dielectric strength of the interlevel dielectric layer decreased, an increased amount of leakage current flowed between the lines or the lines were disconnected. Such a substrate may be made by the method according to any of the preferred embodiments of the present invention described above.
In another preferred embodiment of the present invention, the first material may be a conductive material and the buried structure may be an interconnect. In an alternative preferred embodiment, the first material may be a light blocking material and the buried structure may be a black matrix.
As another alternative, the first material may also be a transparent material and the buried structure may also be a waveguide.
Yet another preferred embodiment of the present invention provides a display device. The display device preferably includes an active-matrix substrate that has been formed by using the conductive or light blocking material and a display medium layer.
In one preferred embodiment of the present invention, the buried structure may be a gate line and the active-matrix substrate may include a reverse staggered TFT that has been formed on the gate line.
In an alternative preferred embodiment, the buried structure may also be a source line and the active-matrix substrate may include a staggered TFT that has been formed on the source line.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.